When 321 patients with resections for colonic neoplasms were prospectively evaluated for changes occurring at the anastomosis, eight different kinds were found (118 abnormalities seen in total). Inflammatory polyps, the most commonly observed abnormality (14.5%), may be misinterpreted as recurrent neoplasia by endoscopy. The majority of inflammatory polyps were discrete, 5- to 15-mm lesions, although diffuse nodularity was occasionally seen. Staples or sutures were visible at 11.3% of the anastomoses. Benign strictures, which developed in 7.1%, occurred primarily after left colonic resection with end-to-end anastomosis. Prominent vessels were occasionally seen at the anastomotic site (3.9%). Recurrent carcinoma at the anastomosis was found in 6 of 116 patients with Dukes B and C tumors (5.2%) and occurred 0.4 to 2.0 years after surgery (mean, 1.2 years). Recurrent carcinoma appeared as ulcerated submucosal lesions, bulky luminal masses, and polypoid lesions. In two patients, mucosal erythema, edema, and friability at the anastomosis were the only endoscopic evidence of underlying carcinoma.